1. Field of Invention
The present invention relates to an optical switch, and particularly relates to an optical switch for switching an optical path by changing a refractive index of the circumference of a light coupling region of a directional coupler
2. Description of Related Art
Among the best-known examples of conventional optical switches for use with optical fibers are optical switches of the waveguide-type, in which a waveguide is formed in an electrooptical crystal such as LiNbO.sub.3 or the like, or in which a waveguide and an electrode are formed in a semiconductor substrate such as GaAs or the like so that a voltage or a current is applied on a portion of the waveguide to change the refractive index of the portion to thereby switch the optical path. As disclosed in the specification of U.S. Pat. No. 4,786,130, an a optical switch is also known in which light is switched from one line to another line by changing the refractive index of the circumference o an optical fiber coupler by heating/cooling the circumferential medium.
However, as a means for changing the refractive index, generally, optical switches using an electrooptical crystal such as LiNbO.sub.3 or the like or a semiconductor such as GaAs or the like generally have a high refractive index, so that there occurs a large loss of energy in the connection of such optical switches with optical fibers. In optical switches which use a semiconductor substrate, there occurs a loss of several decibels. This type of optical switch is therefore not suitable for incorporation into an optical communication line.
Further, in most of the optical switches which use an electrooptical effect as the means for changing the refractive index, the optical switching operation, as a result of its structure, depends so much on the polarization direction of incident light that this polarization must be strictly controlled in order to achieve the desired function. Accordingly, the apparatus is necessarily large in size, which results in a large loss of energy.
Further, in the optical switch which uses the change of the refractive index by heating/cooling the circumferential medium, it is difficult to maintain at the same time both the temperature stability of the operation and the high switching speed. Therefore, high speed operation is limited.